The present invention relates generally to exposure apparatuses for fabricating, for example, semiconductor devices, imaging devices, liquid crystal display devices, thin film magnetic heads, and other micro devices, and more particularly to an exposure apparatus that exposes by using EUV light (extreme ultraviolet light) or X-ray and the like.
Conventionally, reduction projection exposure with UV light have been used for lithography to manufacture fine semiconductor devices like semiconductor memories or logic circuits. The transferable minimum critical dimension in the reduction projection exposure is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture of the projection optical system. Along with recent demands for finer semiconductor devices, shorter ultraviolet light wavelengths have been proposed. As a result, reduction projection exposure apparatuses using EUV light with wavelengths of 15 to 20 nm shorter than that of the ultraviolet light were developed.
However, light absorption by materials increased remarkably in the EUV light's wave range. Therefore, a reflection-type or catoptric optical system is used when an exposure apparatus uses EUV light. Additionally, a reflection reticle (mask) is used instead of a transmission reticle to form the pattern to be transferred. The pattern is formed on a mirror by use of an absorber.
Exposure apparatuses (hereinafter “EUV exposure apparatus”) using EUV light uses a multilayer mirror or an oblique incidence total reflection mirror as a reflective element. These mirrors have large light losses, so the number of mirrors used should be minimized. In cases where carbon-containing molecules, such as hydrocarbons, remains in the space where the optical element has been arranged, carbon will gradually adhere to the surface of the optical element when exposed to EUV light in the EUV exposure apparatus. This causes a problem by decreasing the reflectance of the EUV light due to absorption by the adhered carbon. The air pressure of the space where the optical element is arranged for exposure to EUV light should be less than 10−4 Pa, preferably less than 10−6 Pa to prevent carbon adhering. Therefore, the arrangements of the optical systems, such as the reflection mirror, the reticle, and wafer in the EUV exposure apparatus should be in vacuum state.
Optical elements, such as the reticle or the wafer, absorbs the EUV light (as exposure light) and turns most of the energy from the EUV light into heat in the vacuum state. For example, the reflection reticle forms circuit pattern information according to the differences between the reflected EUV light intensity on the light-reflecting portions and the light-absorbing portions. Heat generation by the EUV light's reflection reticle is large in comparison with a conventional transmission reticle when receiving illumination light (the exposure light) because of partial absorption by the reflection reticle. Moreover, heat is stored in the reticle with very little radiating into the atmosphere because the reflection reticle is located in vacuum state.
For example, Japanese patent application publication (No. 9-92613) discloses a refrigerator that cools the reflection reticle by radiation heat transfer to prevent thermal expansion of the reticle.
A reticle chuck for holding the reticle or a wafer chuck for holding the wafer (hereinafter “chuck”) should hold the reticle or the wafer in the vacuum state. When in the vacuum state, methods for holding the reticle or the wafer by the chuck usually utilize electrostatic suction instead of conventional vacuum suction. The electrostatic suction applies 400V to 800V to an electrode, causing the chuck to suction the reticle or the wafer by charging the surface of the chuck. A leakage current among the electrodes running in the chuck generates heat and causes a temperature rise in the chuck. Because of their arrangement in a vacuum state, the reticle chuck and the wafer chuck hardly diffuse the heat instead heat is stored. For example, Japanese patent application publication (No. 9-306834) relevant to U.S. Pat. No. 6,084,938 discloses a structure in use with a temperature controlling medium or a Peltier device for controlling the temperature of the chuck.
In holding the device on the chuck, a device such as the reticle or the wafer cannot be flat when a particle is wedged between these devices and the chuck because the precision of the projection exposure decreases. Therefore, using a pin chuck with a smaller contact area than the device as the chuck decreases the probability of wedging the particle (see Japanese patent application publication No. 9-306834 and Japanese examined patent publication No. 60-15147 relevant to U.S. Pat. No. 4,213,698).
The chuck needs to be of high rigidity and have the ability to reform the flatness of the optical element. It also needs to be lightweight so as to be moved on a stage in scanning exposure process. Moreover, it needs to have a low coefficient of linear expansion because of the necessity to minimize deformity from heat. Because of the demands explained above, the chuck uses ceramic materials such as silicon carbide (SiC), silicon nitride (SiN), nitride aluminum (AlN) and the like.
However, the chuck used in the EUV exposure apparatus cannot use materials having an ideal low coefficient of thermal expansion because it should keep sufficient electrostatic suction force. The ceramic materials explained before have comparably low coefficients of linear expansion, with coefficient values at almost 1 to 10 ppm.
For example, under the following conditions: 0.01 degree centigrade rise in temperature caused by the leakage current, a 150 mm radius for the wafer chuck, and 3 ppm for its coefficient of linear expansion might shift the wafer's position by 4.5 nm during thermal expansion. A position shift of 4.5 nm by thermal expansion may be a problem because the demand for position accuracy of the wafer chuck in the EUV exposure apparatus is less than a few nanometers.
Object that raises its temperature by absorbing the EUV light (for example, optical elements such as the wafer or the reticle) have coefficient of linear expansion of 2 to 3 nm. So, they expand 3 to 4 nm when the temperature rises by 0.01 degree centigrade. A wafer thickness of, for example, 0.775 mm, has low rigidity. However, friction suctions between the wafer and the chuck surface restricts thermal expansion. So, the actual allowance in temperature rise can be up to about 0.1 degree centigrade.
The reticle consists mainly of glass ceramic and has a low coefficient of linear expansion of 50 ppb. The wafer is exchanged every 30 to 200 seconds and the reticle is exchanged with every scores of wafer exposed. Therefore, the reticle's temperature rise is more than the wafer because it receives more radiation heat transfer. For example, a temperature rise of 1 degree centigrade in the reticle causes a problem with a thermal expansion of approximately 5 nm.
Temperatures of optical elements such as the wafer, the reticle, or the chuck are raised by the heat from the leakage current and/or absorption of EUV light. Further temperature rises comes from their arrangement in vacuum state, which causes the objects to store heat with hardly any heat radiating into the atmosphere. The heat expansion of the optical elements, such as the wafer and the chuck, due to the temperature rise makes transfer of the circuit pattern in exposure imprecise. Thus, the optical element and the chuck need to be cooled in the exposure apparatus.
As the optical element and the chuck are arranged in vacuum state, it is difficult to apply a cooling method using heat transfer by convection in the EUV exposure apparatus. A direct cooling method for cooling the optical element and the chuck directly and use of a coolant seems to be preferable.
However, a precision stage such as the wafer stage for supporting the wafer chuck or the reticle stage for supporting the reticle chuck is supported by an elastic element with low rigidity (for example, a spring). Therefore, with the flow of liquid coolant in the optical element or the chuck, the position of the wafer or the reticle becomes unstable because of vibrations caused by the swirls and/or pulsations generated in the flow paths or pipes. Because the pipe, which is connected to the optical element or the chuck, restricts movement of the precision stage; the position detection and position control responses to the precision stage using an interferometer decreases, and the control for lowering high frequency vibration becomes difficult.
The heat of the optical element and the chuck are hardly transferred between each other via the atmosphere because they are arranged in vacuum state. So, the heat of the optical element and the chuck are mainly exchanged (transferred) with each other via their contact surface. In cases where a pin chuck is used, heat transfer between the pin chuck and the optical element is difficult because the contact area of the pin and the optical element is small. Therefore, there is an optical element cooling problem on the one hand and using the chuck to cool effectively on the other. The problem will be more serious if the contact area of the pin and the optical element is 10% or smaller than the area of the optical element.
Heat generation in a substrate of the optical element or the chuck fluctuates in accordance with the change in suction time of the chuck or intensity of the exposure light when exposing, for example, resists having different sensitivity. Impartial cooling of heat generated in the circuit board of the optical element or the chuck causes fluctuation in heat expansion despite a decrease in the heat, resulting in precise positioning difficulties of the optical elements, such as the wafer or the reticle.
It seems preferable to detect the temperatures of the optical elements, such as the wafer or the reticle and the chuck, to control their temperatures. Temperature detection of the chuck should be more precise than that of the optical element because the temperature rise of the chuck is smaller than that of the optical element. The temperature detection of the optical element should not be affected by the exchangeability of the optical element because of its frequent exchange.